Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22

Questions and Problems • 513

AlP and InSb,(c)GaAs and ZnSe,(d)ZnSe

and CdTe, and(e)CdS and NaCl.

Extrinsic Semiconduction

12.12Ann-type semiconductor is known to have

an electron concentration of 5× 1017 m−^3.

If the electron drift velocity is 350 m/s in an

electric field of 1000 V/m, calculate the con-

ductivity of this material.

12.13 (a)Explain why no hole is generated by the

electron excitation involving a donor impu-

rity atom.(b)Explain why no free electron is

generated by the electron excitation involv-

ing an acceptor impurity atom.

12.14Will each of the following elements act as

a donor or an acceptor when added to the

indicated semiconducting material? Assume

that the impurity elements are substitutio-

nal.

Impurity Semiconductor

Al Si

Cd GaAs

Sb ZnSe

12.15 (a)The room-temperature electrical conduc-

tivity of a silicon specimen is 5.93× 10 −^3

(-m)−^1. The hole concentration is known

to be 7.0× 1017 m−^3. Using the electron and

hole mobilities for silicon in Table 12.3, com-

pute the electron concentration.(b)On the

basis of the result in part (a), is the specimen

intrinsic,n-type extrinsic, orp-type extrinsic?

Why?

12.16The following electrical characteristics have

been determined for both intrinsic andp-type

extrinsic gallium antimonide (GaSb) at room

temperature:

σ(-m)−^1 n(m−^3 ) p(m−^3 )

Intrinsic 8.9× 104 8.7× 1023 8.7× 1023

Extrinsic 2.3× 105 7.6× 1022 1.0× 1025

(p-type)

Calculate electron and hole mobilities.

The Temperature Dependence of

Carrier Concentration

12.17At temperatures near room temperature, the

temperature dependence of the conductivity

for intrinsic germanium is found to equal

σ=CT−^3 /^2 exp

(

−

Eg

2 kT

)

(12.36)

whereCis a temperature-independent con-

stant andTis in Kelvins. Using Equation

12.36, calculate the intrinsic electrical con-

ductivity of germanium at 175◦C.

12.18Using Equation 12.36 and the results of

Problem 12.17, determine the temperature at

which the electrical conductivity of intrinsic

germanium is 40 (-m)−^1.

12.19Estimate the temperature at which GaAs

has an electrical conductivity of 1.6× 10 −^3

(-m)−^1 assuming the temperature depen-

dence for σ of Equation 12.36. The data

shown in Table 12.3 might prove helpful.

Factors That Affect Carrier Mobility

12.20Calculate the room-temperature electrical

conductivity of silicon that has been doped

with 10^23 m−^3 of arsenic atoms.

12.21Estimate the electrical conductivity, at 75◦C,

of silicon that has been doped with 10^22 m−^3

of phosphorus atoms.

The Hall Effect

12.22Some hypothetical metal is known to have

an electrical resistivity of 3.3× 10 −^8 (-m).

Through a specimen of this metal 15 mm thick

is passed a current of 25 A; when a magnetic

field of 0.95 tesla is simultaneously imposed

in a direction perpendicular to that of the cur-

rent, a Hall voltage of –2.4× 10 −^7 V is mea-

sured. Compute(a)the electron mobility for

this metal, and(b)the number of free elec-

trons per cubic meter.

Semiconducting Devices

12.23Briefly describe electron and hole motions in

ap–njunction for forward and reverse biases;

then explain how these lead to rectification.

12.24What are the two functions that a transistor

may perform in an electronic circuit?